1. Field of the Invention
The present invention relates to an active snubber circuit and a power supply circuit, and more particularly, to an active snubber circuit and a power supply circuit capable of absorbing and reducing a surge voltage of a switching element that is used in a switching power supply device and the like.
2. Description of the Related Art
Heretofore, in a flyback converter (RCC) circuit, a snubber circuit is connected between primary coils of a flyback transformer so as to absorb and reduce a surge voltage of a main switching element. The snubber circuit includes a passive-type snubber circuit using an RCD snubber or charge accumulating diode and an active clamp-type snubber circuit (active snubber circuit) using a power semiconductor device.
FIG. 6 shows a related art using the RCD snubber circuit. In FIG. 6, an RCD snubber circuit 101 is connected to a circuit 100 that is referred to as a flyback converter. A main switching element 102 repeatedly performs an on-off operation, and excitation energy, which is accumulated in a transformer 103 when the main switching element 102 performs the on operation, is discharged to supply direct current power to a load when the main switching element 102 performs the off operation.
In FIG. 6, the flyback converter circuit 100 has the main switching element 102 such as MOSFET, a diode 104, a capacitor 106 and a main control circuit 109. In addition, the RCD snubber circuit 101 has a diode 105, a capacitor 107 and a resistance 108. Additionally, the transformer 103 has coils 103a to 103c. The main control circuit 109 is input with a voltage obtained by rectifying and smoothing a voltage of the coil 103b by the diode 104 and the capacitor 106. Since the voltage is a voltage proportional to an output voltage, the main switching element 102 is on-off controlled so as to keep a value of the voltage to be constant.
An operation of the related art will be briefly described. At a time of startup, a startup resistance (not shown) in the main control circuit 109 is used to supply energy for startup from an input power source to the main control circuit 109 so as to start a gate driving of the main switching element 102. When the main switching element 102 is turned on, a voltage of the input power source is applied to the coil 103a of the transformer 103, the main switching element 102 becomes turned on and the excitation energy is accumulated in the coil 103a. Further, when the main switching element 102 is turned off by the main control circuit 109, the excitation energy accumulated in the coil 103a is discharged as electric energy through the coil 103c, which is then rectified and smoothed by a diode 110 and a capacitor 111 for smoothing and supplied to a load 112.
When the excitation energy accumulated in the coil 103a is completely discharged through the coil 103c, a voltage waveform, which is freely vibrated by an excitation inductance of the transformer and a stray capacitance of the transformer and the semiconductor device, is generated at the coil 103a. Then, these on and off operations are repeated. Like this, as the main switching element 102 performs the on and off operations, the electric energy is supplied to the load 112.
When the main switching element 102 is turned off, the energy accumulated in a leakage inductance of the coil 103a of the transformer 103 is absorbed on the capacitor 107 of the RCD snubber circuit 101 and is consumed in the resistance 108. Thus, a surge voltage that is applied to the main switching element 102 is suppressed.
FIG. 7 schematically shows a waveform of a voltage applied between a source and a drain of the main switching element 102 of the related art shown in FIG. 6.
As shown in FIG. 7, when the RCD snubber circuit 107 is used, the surge voltage cannot be sufficiently suppressed, so that EMI (Electromagnetic Interference) is increased.
JP-A-2006-129548 discloses a related-art power supply circuit using an active snubber circuit. The related-art power supply circuit disclosed in JP-A-2006-129548 is a power supply circuit that uses current resonance to perform power delivery, has a series circuit of a main switching element and a sub-switching element connected both ends of a direct current power source, a transformer, main and sub control circuits and a series circuit of a condenser and an inductance between mutual connection points of the main switching element and the sub-switching element. The related-art power supply circuit disclosed in JPA-2006-129548 alternately turns on and off the main switching element and the sub-switching element by the respective control circuits, thereby rectifying, smoothing and supplying a voltage generated at a secondary coil to a load. A primary-side coil of the transformer and the main control circuit turn on and off the main switching element so that the direct current voltage supplied to the load is constant with a voltage of a primary auxiliary coil as a signal voltage, and the sub-control circuit turns on the sub-switching element when a both-end voltage of the sub-switching element is lowered to a reference voltage or lower.
However, when the snubber circuit is configured in a passive manner, the surge voltage is changed into heat energy, which is then consumed. Thus, each part is enlarged and power supply efficiency is decreased. Alternatively, when the snubber circuit is configured in an active clamp manner, the surge voltage can be power-regenerated, so that the efficiency is not decreased. However, it is difficult to acquire an optimal on-off timing of a power element.
In JP-A-2006-129548, the both-end voltage of the sub-switching element is detected, and the sub-switching element is turned on when the voltage is lowered to a reference voltage or lower. Hence, loss is always caused due to a resistance detecting the both-end voltage of the sub-switching element, so that power consumption is increased during the standby such as unload or light load. JP-A-2006-129548 also discloses a method of adding a coil for signal to the transformer to acquire an on-off timing of the sub-switching element. However, since an extra tap is required for the transformer, spaces between the respective coils are necessary, so that a structure of the transformer is complicated and an outward size of the transformer should be enlarged.